Although that claim is a bit simplistic, it is not without claim. You might start here http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2814%2961

Although that claim is a bit simplistic, it is not without claim. You might start here http://www.thelancet.com/journals/lancet/article/PIIS0140-6736%2814%2961839-0/abstract http://www.thelancet.com/journals/lancet/article/PIIS0140-67... If humans or any living thing didn't have the ability to fight off disease life wouldn't exist. The degree to which anything can fight off an illness is a combination of genetics and environment which includes its overall health status.